This invention relates to a method for making a printing plate, wherein a toner image is electrophotographically formed by using a wet-type developer on an original printing plate comprising an electroconductive support and an organic photoconductive compound layer disposed thereon, then the image is fixed, and the non-image areas are removed by etching with an alkaline aqueous etching solution, leaving behind the toner image intact.
It is known that as the photosensitive material of a printing plate using electrophotographic image forming, use is made of a photosensitive element comprising paper, film, or aluminum sheet and, provided thereon, a coating layer of an inorganic photoconductive powder, such as zinc oxide or cadmium sulfide, dispersed in a suitable binder. To form an image and to convert the element to a printing plate by the electrophotographic technique, the element is subjected to corona discharge, exposure, and toner development, the resulting toner image being receptive to a greasy printing ink and the non-image areas becoming hydrophilic after hydrophilizing treatment. Although the toner image is obtained by either dry development or wet development, the latter is generally preferred because of better resolution of the image. In such a printing plate, however, since a large quantity of an inorganic photoconductive powder together with a binder is coated on a support in order to increase the sensitivity, it follows that a certain limitation is posed upon the surface smoothness, strength of the coating film, and the adhesiveness between the coating film and the support; with respect to the retention of sufficient hydrophilicity of the non-image areas there is a problem arising from the strength of the coating film. For these reasons, the printing plate is unsuitable for the long-run printing exceeding 10,000 copies, though it is satisfactory for the short-run printing.
There are also known many printing plates carrying organic photoconductive compounds, which are adaptable to electrophotographic treatment. As examples of such printing plates, mention may be made of those disclosed in Japanese Patent Publication Nos. 17,162/62, 6,961/63, 2,426/66 and 39,405/71; Japanese Patent application "Kokai" (Laid-open) Nos. 19,509/75, 19,510/75, 145,538/79, 89,801/79, 34,632/79, 19,803/79 and 105,244/80. In the printing plates disclosed by these patents, the photosensitive elements comprise electroconductive supports such as aluminum sheet and, disposed thereon, a coating of photoconductive compounds using a binder soluble in alkalis and/or alcohols such as styrene-maleic anhydride copolymer, vinyl acetate-crotonic acid copolymer, vinyl acetate-maleic anhydride copolymer, or phenolic resin, and the toner images are formed by subjecting the photosensitive elements to corona discharge, exposure, and toner development, said toner images serving as a resist layer in the subsequent removal of the coating layer in non-image areas by treating with an etching solution containing an alkali or alcohol as major component, thereby to produce a printing plate comprising the unremoved image areas and the non-image areas of hydrophilic surface of the aluminum support. A commercial product based on this principle has been offerred for practical use under the trade name of "Elefasol" by Kalle Co. The Elefasol system, however, utilizes a dry developer in forming the toner image and, as a consequence, is insufficient in resolving power. Even if use is made of a dry developer of finely subdivided particles, there is obtained a printing plate having a resolving power of only 3-5 lines/mm. As contrasted, the toner image formed by a wet developer has a resolution of 15-20 lines/mm. Although the toner image formed by the dry developer is inferior in resolving power owing to a larger size of toner particles, it has an advantage in that owing to a larger amount of fixed particles the thick toner image acts as a resist in the etching of non-image areas with an alkali and/or an alcohol, thus preventing, in most cases, the image areas from being etched. To the contrary, although superior in resolution and sharpness, the toner image formed by the wet developer is considerably smaller in thickness than that formed by the dry developer, thus detracting much from the effectiveness of toner image as a resist. As a consequence, the resolving power of the printing plate before etching is no longer retained after the etching. Therefore, it is also difficult to obtain a printing master of high resolving power even by the wet development.
Another problem generally accompanying the wet developer is the stability of dispersion. Since the wet developer is a dispersion of a resin, a dye or pigment, a charge regulator, and others in an organic solvent of low dielectric constant and high insulating property, the dispersion stability is an extremely important factor for the manufacturers of wet developers which requires utmost endeavor for the improvement. An especially difficult problem to be solved during the developmental stage of the present wet developer is to find a way of controlling such contradictory tendencies that a resin composition having an improved dispersion stability tends to be inferior in resist property, while that having an improved resist property tends to be inferior in dispersion stability.